This invention relates generally to the analysis of the composition of materials by means of spectroscopy and is particularly directed to a gas chromatographic/matrix-isolation device for the spectroscopic analysis of gas samples.
Gas chromatographic separations have been useful fundamental tools of chemical research and analysis for some time. Their usefulness is greatly enhanced when the separated components can be conveniently and promptly analyzed. The matrix-isolation technique for presenting samples for spectroscopic examination is also of considerable value in obtaining precise analysis of samples including specific structural information about molecular construction through high-resolution infrared analyses. The technique of matrix-isolation spectroscopy involves the simultaneous condensation of a gaseous sample in an excess of inert gas to form a solid matrix in which sample molecules are isolated from one another. The technique is powerful with respect to both the variety of species that can be studied and the quality of the spectra that can be obtained. Typically, a particular and distinct sample material is entrapped within a frozen matrix of an inert substance such as argon or krypton gas. These systems are typically maintained at very low temperatures, such as 10.degree.-20.degree. K. This technique permits the retention of the sample in a neutral and noncontaminating matrix material over an extended period of time. As a result, high resolution infrared and other spectroscopic types of analyses are available with this approach.
Although the cryogenic matrix isolation technique has been widely practiced in the theoretical study of molecules, it has not been widely used in analytical chemistry laboratories as a routine method for the spectroscopic identification of unknown compounds. This is primarily because this technique in the past has been slow and cumbersome and generally permitted only one or two compounds to be analyzed per run. This has, of course, been unfortunate because when used in combination with gas chromatography, which is a highly efficient and widely practiced method of separating complex mixtures of compounds, the matrix isolation technique is a powerful analytic tool.
To successfully combine these two methods and obtain matrix-isolation infrared spectra of the components eluted from a gas chromatograph, provision must first be made for collecting several components in rapid sequence. Secondly, for a given rate of elution of sample from the chromatograph, the rate of flow and condensation of the matrix-forming gas must be appropriate to maintain proper dilution of the sample in the matrix. Controlling these conditions for optimum spectral analysis has proven difficult and limited the extent of use and acceptance of the matrix-isolation technique.
One approach utilizing gas chromatography/infrared matrix-isolation spectrometry is disclosed in U.S. Pat. No. 4,158,772 issued to the present inventor and shown in schematic form in FIG. 1. In this approach, a vacuum chamber 11 contains a specular carousel 13 which may either have a plurality of sample surfaces (eight are shown in FIG. 1) or may be circular in cross section and provide a continuous sample surface. Also shown enclosed within the vacuum chamber are two concave mirrors 15, 17 aligned toward one of the sample surfaces of carousel 13. Two lenses or optical windows 19, 21 are installed within the vacuum chamber wall and aligned to admit and transmit light to and from the vacuum chamber 11. A beam of light, e.g., in the infrared (IR) spectrum, is provided by a light source and interferometer (or monochrometer) 23 and transmitted into the vacuum chamber through, for instance, mirrors 25, 29 and 27. Similarly arranged mirrors, for instance 31, 35 and 33, direct the return light beam once it has transitted the sample material into a detector within spectrometer 37 for analysis of the spectra of the sample material. The mirrors 15, 17 have spherical or elipsoidal surfaces for directing and focusing a light beam between the optical windows 19, 21 and the sample on a reflecting surface of carousel 13. Gas samples for analysis are provided from a source 39 such as a gas chromatography unit through inlet 38 to the sample block surfaces of carousel 13. A cryostat (not shown) cools the carousel 13 so as to freeze the gaseous sample material onto an appropriate sample block surface and a diffusion pump (not shown) is connected to the vacuum chamber 11 for maintaining a vacuum therein.
Although capable of providing accurate spectrographic data, this approach, as well as other prior art systems, suffers from various limitations which precluded their widespread acceptance and general use. One limitation involves the long effective focal length of the reflector configuration utilized in the system. This results in a more diffuse, larger image and reduced measurement sensitivity. In addition, the various reflectors in the system are all independently mounted thus making it substantially more difficult to accurately focus the incident light beam. For example, reflectors 15 and 17, which are positioned within the vacuum chamber 11, must not only be aligned with optical windows 19 and 21 mounted in a wall of the vacuum chamber, but also with mirrors 25, 27, 29, 31, 33 and 35. This number of reflectors arranged in a generally linear alignment makes precise beam positioning extremely difficult. Finally, the geometry of the optics requires the incident and reflected beams to circumscribe the vacuum chamber 11. In addition, the sample source 39 provides a gaseous sample to carousel 13 through the same vacuum chamber wall through which the incident and reflected beams are transmitted. The opposite side of the carousel 13 is nearly entirely enclosed by concave mirrors 15 and 17. Thus, visual access to the deposited sample for monitoring the aforementioned important conditions is a matrix-isolation system discussed above is virtually impossible in this system. The measurement accuracy of prior art gas chromatography/infrared matrix-isolation spectrometry systems, including that disclosed in the aforementioned patent, is therefore inherently limited by system configuration.
The gas chromatography/matrix-isolation apparatus of the present invention overcomes the aforementioned limitations of the prior art by providing a system in which the incident electromagnetic beam may be precisely positioned relative to the sample under investigation and the formation of the sample may be closely monitored to provide optimum sample deposition.